Method of preparing a metal alloy part

ABSTRACT

A method of converting an alloy comprising a majority of titanium, the method comprising:
         a step of fabricating an ingot ( 1 );   steps of a first type (A, B, C) of plastically deforming the alloy at a temperature higher than the β transus temperature Tβ;   steps of a second type (A′, B′) of plastically deforming the alloy at a temperature lower than the β transus temperature Tβ.       

     These step of the first and second types (A, A′, B, B′, C) are performed in the following sequence:
         a first step of the first type (A) at a first temperature T 1;      a first step of the second type (A′);   a second step of the first type (B) at a second temperature T 2  lower than T 1;      a second step of the second type (B′); and   a third step of the first type (C) at a third temperature T 3  lower than T 2.

The invention relates to a method of converting an alloy containing a majority of titanium.

BACKGROUND OF THE INVENTION

More particularly, the invention relates to a method of converting an alloy that comprises, in percentage by weight of alloy, a majority of titanium, the alloy presenting a β transus temperature beyond which a transition is observed from a phase alloy structures to β phase alloy structures, the method comprising:

-   -   a step of fabricating an ingot made of said alloy;     -   at least first, second, and third steps of a first type         consisting in plastically deforming the alloy from said ingot         while it is at a current temperature strictly higher than the β         transus temperature; and     -   at least first and second steps of a second type consisting in         plastically deforming the alloy from said ingot while it is at a         current temperature strictly lower than the β transus         temperature.

In percentage by weight of alloy, titanium alloys contain a majority of titanium and in particular at least 60% by weight of the alloy is constituted by titanium.

It has been found that parts belonging to a single batch of parts obtained from the same alloy present non-uniform mechanical strength.

For production quality reasons, it is desirable for similar parts obtained from the same titanium alloy to present mechanical strength that is uniform.

OBJECT OF THE INVENTION

An object of the invention is to provide a method of converting an alloy that contains a majority of titanium in percentage by weight of the alloy, the method seeking to improve the quality of parts produced using the alloy that has been converted by the method of the invention.

SUMMARY OF THE INVENTION

To this end, the invention provides a method of converting an alloy that comprises, in percentage by weight of alloy, a majority of titanium, the alloy presenting a β transus temperature beyond which a transition is observed from α phase alloy structures to β phase alloy structures, the method comprising:

-   -   a step of fabricating an ingot made of said alloy;     -   at least first, second, and third steps of a first type A, B, C         consisting in plastically deforming the alloy from said ingot         while it is at a current temperature strictly higher than the β         transus temperature Tβ; and     -   at least first and second steps of a second type A′, B′         consisting in plastically deforming the alloy from said ingot         while it is at a current temperature strictly lower than the β         transus temperature T.

The conversion method of the invention is essentially characterized in that the steps of the first and second types A, A′, B, B′, C are applied in the following sequence:

-   -   performing the first step of the first type A while the alloy is         at a first temperature T1; followed by     -   performing the first step of the second type A′; followed by     -   performing the second step of the first type B while the alloy         is at a second temperature T2 strictly lower than said first         temperature T1; followed by     -   performing the second step of the second type B′; followed by     -   performing the third step of the first type C while the alloy is         at a third temperature T3 strictly lower than said second         temperature T2.

In order to understand the invention, the β transus temperature Tβ is the temperature above which a transition is observed in at least some of the structures of the alloy going from α phase to alloy structures of β phase.

The alloy portion in α phase presents a compact hexagonal crystallographic microstructure.

The alloy portion in β phase presents a body centered cubic crystallographic microstructure.

Thus, on passing above the β transus temperature, it is found that alloy portions that were in compact hexagonal form transform into body centered cubic alloy portions. The sequence of steps in the method of the invention combines heat treatments with plastic mechanical deformation operations of the alloy, which treatment and operations are performed in such a manner as to make the internal microstructure of the alloy more uniform by progressively making the size of the crystals/grains constituting the alloy more uniform.

Thus, parts belonging to the same batch of parts produced from a titanium alloy converted by the method of the invention present characteristics that are made more uniform from the point of view of microstructure, from the point of the size distribution of β phase grains contained in the alloy, and from the point of view of chemical composition (the chemical species are better distributed in the alloy that has been converted by the method of the invention than they were in the ingot prior to performing the various steps of the method of the invention).

Thus, the overall quality of the batch of parts is improved since the alloy constituting the parts presents characteristics that are uniform among the parts of the batch.

When forming the ingot, which weighs several (metric) tonnes, an ingot typically weighing 3 tonnes to 7 tonnes and being more than 2 meters (m) tall, it is observed that the ingot presents stratification such that the bottom and central portions of the ingot present elongate crystals of mean length and section that are much greater than the mean length and section of crystals to be found in the top portion of the ingot.

The first step of the first type A is performed at a first temperature T1 higher the β transus temperature and that serves to transform at least a portion of the crystallographic structures of the alloy that are in α phase to crystallographic structures in β phase. The mechanical deformation/plastic deformation of large alloy grains in β phase leads to these large grains in β phase being broken so that they recrystallize as smaller grains, still in β phase. This thus begins making the alloy of the ingot more uniform in terms of the nature of the α and β phases present in the alloy and in terms of β grain size.

The first step of the second type A′, which is performed below the β transus temperature, specifically at a temperature T4, in order to conserve the nature of the α and β phases present in the material while applying mechanical plastic deformation to the alloy serves to create/accumulate internal mechanical stresses within the alloy and around the β phase grains.

During the following step, which is the second step of the first type B, the temperature of the alloy is raised above the β transus temperature until it reaches a second temperature T2 that is strictly lower than the first temperature T1. During this step B, the mechanical stresses that have accumulated around the β phase grains during the first step of the second type A′ once more give rise to breaks/dislocations of the β grains that are of larger size and that are subjected to greater stresses. The effect of these dislocations is to encourage the largest β grains of the alloy to recrystallize. This second step of the first type B is a recrystallization step, that serves to prepare a first uniformization of β grain size by accumulating dislocations in the larger grains, or in grains that are less well oriented relative to the main part of the microstructure.

During the following step, which is the second step of the second type B′, the temperature of the alloy is lowered once more so that it has a current temperature T4 that is lower than the β transus temperature Tβ, and plastic deformation is applied once more to the alloy so as to create new mechanical stresses in the alloy and around the β phase grains.

Since this step B′ is performed below the β transus temperature, the α and β phases of the grains present in the alloy are conserved and only mechanical stresses are generated around the most non-uniform β grains relative to the microstructure.

During the following step, which is the third step of the first type C, the temperature of the alloy is raised once more so that it has a current temperature, referred to as the third temperature T3, that is higher than the β transus temperature, but strictly lower than the second temperature T2 that it reached the second step of the first type B. During this third step of the first type C, the mechanical stresses accumulated around the β grains during the second step of the second type B′ lead once more to breaks/dislocations of the β grains that are of the largest size and that are subjected to the greatest stresses. The effect of these new dislocations continues to encourage recrystallization of the β grains that contain the most dislocations. The alloy is thus once more made more uniform.

The fact that the first, second, and third steps of the first type are performed at progressively lowering temperatures while nevertheless remaining above the β transus temperature makes it possible to create progressively finer dislocations in order to encourage the precipitation of the most non-uniform grains of β phase alloy. All of these steps of the conversion method of the invention serve to make the crystallographic structure of the alloy more uniform both in terms of the size distribution of α phase and of β phase grains in the alloy, and also in terms of the dimensions of these respective grains.

The alloy as converted in this way presents mechanical characteristics that are more uniform, thereby enabling characteristics to be made more uniform as a function of the directions intended for metal parts obtained from the alloy.

In a preferred implementation of the method of the invention, it is ensured that:

-   -   the first temperature T1 is higher than the β transus         temperature Tβ by at least 200° C. and at most 300° C.;     -   the second temperature T2 is higher than the β transus         temperature Tβ by at least 100° C. and at most 200° C.;     -   the third temperature T3 is higher than the β transus         temperature Tβ by at least 50° C. and at most 150° C.

The facts:

-   -   firstly of progressively limiting the difference between the β         transus temperature Tβ and the successive temperatures T1, T2,         T3 used for the first, second, and third steps of the first type         A, B, C; while     -   also ensuring that a limit temperature Tlim above the β transus         temperature Tβ is not exceeded;

make it possible to avoid the risks of neighboring β phase grains recombining to make a single large β phase grain, which would go against the looked-for effect of making the alloy more uniform.

In a preferred implementation of the invention in combination with any of the above-described implementations, it is ensured that each plastic deformation performed during a step of the second type A′, B′, C′ is such as to tend to reverse at least in part the effect of the deformation applied to the alloy during the step of the first type immediately preceding said step of the second type.

By reversing the effect of deformation, it should be understood that at least one of the deformations to which the alloy has been subjected is reversed. Thus, if a first deformation operation leads to a shortening in the length of the billet made up of the alloy, then the deformation operation that reverses the effect of the first deformation operation needs to be performed in such a manner as to increase the length of the billet.

During a step of the second type A′, B′, C′, reversing the effect of the deformation applied during the preceding step of the first type A, B, or C serves to increase the capacity for deformation that can be imparted during a subsequent step of the first type. If deformation were not performed for reversing at least in part the effect of the deformation performed during a step of the first type, then the capacity of the alloy for deforming during the following step of the first type would be much more limited. The deformation operations performed between two successive steps of the first type A, B, C would be cumulative and would lead to deformation causing complete local breakage of the alloy.

Consequently, reversing the effect of deformation makes it possible to limit the deleterious effects associated with multiple deformation operations performing during the steps of the first type.

In a preferred implementation, each of the plastic deformation operations performed during the steps of the first type are operations of deformation by compressing the alloy in an alloy compression direction that is common to all of the steps of the first type, each of these plastic deformation operations during the steps of the first type having an effect of shortening the length Lx of the alloy.

The length Lx of the alloy is the longest dimension of the alloy or alloy block that is subjected to deformation. Whether the alloy is in the form of an ingot or of a billet, this length Lx of the alloy is always the longest dimension that can be measured on the alloy and this length Lx is thus a running length of the alloy as measured prior to subjecting the alloy to a new deformation step.

Thus, during the steps of the first type, the alloy tends to be compacted by shortening its greatest running dimension Lx. This type of deformation performed at a temperature higher than Tβ weakens the alloy less than deformation tending to stretch the alloy.

Preferably, each of the plastic deformation operations performed during the steps of the second type A′, B′, C′ are operations of deforming the alloy by compression oriented in such a manner as to obtain on each step of the second type an increase in the length Lx of the alloy.

Typically, the plastic deformation operations performed during the steps of the second type are obtained by compressing the alloy in compression directions that are perpendicular to the alloy compression direction common to all of the steps of the first type.

BRIEF DESCRIPTION OF THE DRAWING

Other characteristics and advantages of the invention appear clearly from the following description given by way of non-limiting indication and with reference to the drawing, in which FIG. 1 shows the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of the method of the invention is to convert a titanium alloy that is initially in the form of an ingot, the conversion method serving to make the microstructure characteristics of the alloy more uniform.

The alloy converted by the conversion method of the invention is in the form of one or more billets. The alloy as obtained in this way in the form of billets is then successively:

-   -   forged to generate particular shapes needed for the final part         that is preferably a large part of landing gear, such as a rod         or a truck; then     -   machined in order to remove some of the alloy present on the         forging; then optionally     -   subjected to solution heat treatment and quenched in water or         air; and then     -   thermally aged in order to be hardened, the alloy as aged in         this way being a quasi-β alloy containing nodules of primary         alpha phase alloy between the β grains, together with a         precipitate of secondary alpha inside the β grains.

Although the invention relates essentially to the conversion method of the invention, it can also relate to a method of producing a part such as a rod, a truck, or a strut of aircraft landing gear, or any part of size comparable to a landing gear (longer than 1 m), fabricated from an alloy that has been converted in accordance with the alloy conversion method of the invention.

In addition to the alloy conversion method of the invention, the production method includes the above-mentioned subsequent steps of forging, machining, and aging in order to obtain a quasi-finished landing gear part of large size such as a landing gear rod, truck, or strut.

The present description continues with the conversion method of the invention.

The first step of the conversion method of the invention consists in producing an alloy that comprises a majority of titanium in percentage by weight of the alloy. The alloy is selected to present a β transus temperature Tβ lying in the range 800° C. to 950° C., and preferably of 900° C.

More particularly, the alloy is selected from the group of alloys comprising:

-   -   a first alloy (Ti 10-2-3) comprising the following elements in         percentages by weight:

aluminum, Al 2.6%-3.4% carbon, C ≦0.050% hydrogen, H ≦0.015% iron, Fe 1.6%-2.2% nitrogen, N ≦0.050% oxygen, O ≦0.013% titanium, Ti   83%-86.8% vanadium, V 9.0%-11% 

-   -   a second alloy of type (Ti 5-5-5-3) comprising, the following         elements in percentages by weight:

iron, Fe 0.5%-1.5% carbon, C maximum 0.1% silicon, Si maximum 0.15% chromium, Cr 0.5%-1.5% molybdenum, Mo   4%-5.5% vanadium, V   4%-5.5% nitrogen, N maximum 0.05% titanium, Ti 79.4%-86.3% aluminum, Al 4.4%-5.7% zirconium, Zr maximum 0.3% oxygen, O maximum 0.18% hydrogen, H maximum 0.15% impurities 0.3%

-   -   a third alloy of type (Ti 5-5-5-3-1) comprising the following         elements in percentages by weight:

iron, Fe 0.5%-1.5% carbon, C maximum 0.1% silicon, Si maximum 0.15% chromium, Cr 0.5%-1.5% molybdenum, Mo   4%-5.5% vanadium, V   4%-5.5% nitrogen, N maximum 0.05% titanium, Ti 79.4%-86.3% aluminum, Al 4.4%-5.7% zirconium, Zr   1% oxygen, O maximum 0.18% hydrogen, H maximum 0.15% impurities: 0.3%

-   -   a fourth alloy of type (Ti18) described in patent Document GB 2         470 613 A and comprising the following elements, in percentages         by weight:

aluminum, Al 5.3%-5.7% vanadium, V 4.8%-5.2% iron, Fe 0.7%-0.9% molybdenum, Mo 4.6%-5.3% chromium, Cr 2.0%-2.5% oxygen, O 0.12%-0.16% the balance being at least titanium and impurities; and

-   -   a fifth alloy comprising the following elements, in percentages         by weight:

titanium, Ti at least 84% aluminum, Al 4%-7.5% oxygen, O at least 0.1% carbon, C at least 0.01% at least one element selected from vanadium, molybdenum, chromium, and iron, this fifth alloy also including hafnium and zirconium in addition at a percentage by weight of at least 0.1%.

The fifth alloy is particularly suitable for being converted using the method of the invention since it presents a β transus temperature Tβ lying in the range 800° C. to 950° C., and more particularly a β transus temperature Tβ=900° C.

More particularly, this fifth alloy includes in percentages by weight at least 84% titanium and at least the following elements:

aluminum, Al 4.0%-7.5% vanadium, V 3.5%-5.5% molybdenum, Mo 4.5%-7.5% chromium, Cr 1.8%-3.6% iron, Fe 0.2%-0.5% hafnium, Hf 0.1%-1.1% oxygen, O 0.1%-0.3% carbon, C 0.01%-0.2% 

Each of these titanium alloys presents β transus temperature β that is specific thereto.

Typically, the temperature of the preferred fifth alloy is β=900° C.

As mentioned above, the β transus temperature is the temperature beyond which a transition is observed from a phase alloy structures to β phase structures.

The alloy as produced in this way is cast in order to form an ingot 1 of said alloy.

As can be seen in FIG. 1, the conversion method of the invention comprises:

-   -   at least first, second, and third steps of a first type A, B, C         consisting in plastically deforming the alloy from the ingot         while it is at a current temperature strictly higher than the β         transus temperature β and less than a limit temperature         Tlim=β+300° C.; and     -   at least first and second steps of a second type A′, B′         consisting in plastically deforming the alloy from said ingot         while it is at a current temperature strictly lower than the β         transus temperature, Tβ.

In the present example, the method includes a third step of the second type C′.

These steps of the first and second types A, A′, B, B′, C are performed for a given portion of alloy by following the sequence that consists in:

-   -   performing the first step of the first type A while the alloy is         at a first temperature T1; then     -   performing the first step of the second type A′ while the alloy         is at a temperature T4, referred to as the fourth temperature;         then     -   performing the step of the second type of the second type B         while the alloy is at a second temperature T2 strictly lower         than said first temperature T1; then     -   performing the second step of the second type B′ while the alloy         is at T4; then     -   performing the third step of the first type C while the alloy is         at a third temperature T3 strictly lower than said second         temperature T2; and then     -   performing the third step of the second type C′ while the alloy         is at T4.

Typically, T1 is defined by (β+200° C.)<T1<(Tβ+300° C.); T2 is defined by T2<T1 and (Tβ+100° C.)<T2<(Tβ+200° C.); T3 is defined by T3<T2<T1 and (Tβ+50° C.)<T3<(Tβ+150° C.); and T4, which is the fourth temperature used during each of the steps of the second type, is defined by (Tβ−65° C.)<T4<(Tβ−35° C.) or preferably type (Tβ−55° C.)<T4<(Tβ−45° C.). In other words, each of the steps of the second type is performed at the fourth temperature T4 lying between the β transus temperature (Tβ) minus 50° C. to within plus or minus 15° C., and preferably to within plus or minus 5° C. In FIG. 1, Tβ=800° C., T1=1100° C., T2=1000° C., T3=900° C., and T4=750° C.

These temperatures T1, T2, T3, T4 are satisfied if they are within ±15° C. of the specific temperature, and preferably within ±5° C. of that temperature. The selected fourth temperature T4 makes it possible to conserve the α and β phases present in the alloy without accumulating excessive stresses around the β grains.

Although the temperatures at which these steps of the second type A′, B′, C′ are performed are described as being identical, it is possible for them to be different from one another.

Before the first step of the first type A, the ingot made of the alloy presents a running length Lx defining a main axis X-X of the alloy.

In all of the steps of the first type A, B, C, the direction in which the alloy is compressed is oriented parallel to this main axis of the alloy, and more particularly parallel to this length of the ingot.

The directions in which the alloy is compressed during the steps of the second type A′, B′, C′ are perpendicular to the length of the alloy, i.e. perpendicular to the main axis X-X.

Typically, the compressions performed during the steps of the first type A, B, C are performed by placing the ingot between the jaws of a press that are moved towards each other in a direction parallel to the length of the ingot.

Typically, the compressions performed during the steps of the second type A′, B′, C′ are obtained by flattening the alloy between optionally shaped tools placed facing each other in order to reduce the section of the alloy and thus progressively lengthen the alloy. The deformations performed during the first step of the first type A comprises at least one upsetting operation R shortening the length Lx of the alloy by 20% to 30% of the length Lx of the alloy measured before performing this first step of the first type A.

The deformation performed during the second step of the first type B also comprises an upsetting operation R shortening the length Lx of the alloy by 20% to 30% of the length Lx of the alloy measured after performing the first step of the second type A′ and before performing the second step of the first type B.

The deformation performed during the third step of the first type C also comprises an upsetting operation R shortening the length Lx of the alloy by 15% to 20% of the length Lx of the alloy measured after performing the second step of the second type B′ and before performing the third step of the first type C.

An upsetting operation R is an operation of compressing the alloy along its length Lx, i.e. along the axis X-X of the alloy.

The deformation E1 performed during the first step of the second type A′ is performed so as to increase the length Lx of the alloy by 20% to 30% of the length Lx of the alloy as measured after performing the first step of the first type A and before increasing the length Lx during the first step of the second type A′.

The deformation E4 performed during the step of the second type B′ is adapted to increase the length Lx of the alloy by 20% to 30% of the length Lx of the alloy as measured after performing the second step of the first type B and before increasing the length Lx during this second step of the second type B′.

After the third step of the first type C, a third step of the second type C′ is performed serving to give the alloy a shape and dimensions that are suitable for subsequent forging of the alloy in order to obtain a forged part.

This third step of the second type C′ may be adapted to increase the length Lx of the alloy by at least 30% of the length Lx of the alloy as measured after performing the third step of the first type C and before increasing the length Lx during the third step of the second type C′.

It should be observed that after the second step of the first type B and before the third step of the second type C′, and preferably between the steps B′ and C, a step X is performed of cutting the alloy in a transverse plane so as to obtain two elongate portions in the form of bars that are referred to as billets 1′ and 1″.

Ideally, these portions/billets 1′, 1″ are identical in shape. The shape of a billet that is to form a large part of an aircraft landing gear is substantially a right cylinder of length lying in the range 2 m to 3 m and of diameter lying in the range 0.4 m to 0.5 m.

Before performing the first step of the first type A, the alloy ingot is originally in the shape of a right cylinder of length lying in the range 3 m to 5 m and of diameter lying in the range 0.6 m to 1.2 m.

The volume of the two billets 1′, 1″ is less than the volume of the ingot, which means that a fraction of the alloy has been lost during the various steps of the alloy conversion method of the invention.

In the present example:

-   -   in step A, an upsetting operation R1 is performed followed by a         stretching operation E1;     -   in step A′, an upsetting operation R2 is performed followed by a         stretching operation E2;     -   in step B, an upsetting operation R3 is performed followed by a         stretching operation E3;     -   in step B′, an upsetting operation R4 is performed followed by a         stretching operation E4;     -   in step C, an upsetting operation R5 is performed followed by a         stretching operation E5; and     -   in step C′, an upsetting operation R6 is performed followed by a         stretching operation E6, which produces the finished billet 1′         ready for forging.

These stretching operations E1, E2, E3, E4, E5, and E6 are operations of lengthening the running length of the alloy Lx obtained by compressing the alloy laterally, and not by traction.

The billet 1′ at the end of the method is made up of a converted alloy in which the microstructure has been made more uniform at least in terms of the dimensions of β phase grains and the distribution of those grains within the alloy in comparison with the microstructure that is observed before performing step A of the method.

Although the method of the invention is described as having three steps of the first type and three steps of the second type, it should be observed that it could also have a larger number of steps of the first step and a larger number of steps of the second type.

Whatever the number of steps of the second type that are performed, it is preferable to ensure that at least one step of the second type performed between two successive steps of the first type. 

1. A method of converting an alloy that comprises, in percentage by weight of alloy, a majority of titanium, the alloy presenting a β transus temperature beyond which a transition is observed from α phase alloy structures to β phase alloy structures, the method comprising: a step of fabricating an ingot (1) made of said alloy; at least first, second, and third steps of a first type (A, B, C) consisting in plastically deforming the alloy from said ingot while it is at a current temperature strictly higher than the β transus temperature (Tβ); and at least first and second steps of a second type (A′, B′) consisting in plastically deforming the alloy from said ingot while it is at a current temperature strictly lower than the β transus temperature (Tβ); the method being characterized in that the steps of the first and second types (A, A′, B, B′, C) are applied in the following sequence: performing the first step of the first type (A) while the alloy is at a first temperature (T1); followed by performing the first step of the second type (A′); followed by performing the second step of the first type (B) while the alloy is at a second temperature (T2) strictly lower than said first temperature (T1); followed by performing the second step of the second type (B′); followed by performing the third step of the first type (C) while the alloy is at a third temperature (T3) strictly lower than said second temperature (T2).
 2. An alloy conversion method according to claim 1, wherein: the first temperature (T1) is higher than the β transus temperature (Tβ) by at least 200° C. and at most 300° C.; the second temperature (T2) is higher than the β transus temperature (Tβ) by at least 100° C. and at most 200° C.; the third temperature (T3) is higher than the β transus temperature (Tβ) by at least 50° C. and at most 150° C.
 3. An alloy conversion method according to claim 1, wherein each plastic deformation performed during a step of the second type (A′, B′) is such as to tend to reverse at least in part the effect of the deformation applied to the alloy during the step of the first type preceding said step of the second type.
 4. An alloy conversion method according to claim 1, wherein each of the plastic deformation operations performed during the steps of the first type are operations of deformation by compressing the alloy in an alloy compression direction that is common to all of the steps of the first type, each of these plastic deformation operations during the steps of the first type having an effect of shortening the length (Lx) of the alloy.
 5. An alloy conversion method according to claim 4, wherein each of the plastic deformation operations performed during the steps of the second type are operations of deforming the alloy by compression oriented in such a manner as to obtain on each step of the second type an increase in the length (Lx) of the alloy.
 6. An alloy conversion method according to claim 5, wherein the deformation (R1) performed during the first step of the first type (A) is adapted to shorten the length (Lx) of the alloy by 20% to 30% of the length (Lx) of the alloy measured before performing this first step of the first type (A).
 7. An alloy conversion method according to claim 6, wherein the deformation (R3) performed during the second step of the first type (B) is adapted to shorten the length (Lx) of the alloy by 20% to 30% of the length (Lx) of the alloy measured after performing this first step of the second type (A′) and before performing this second step of the first type (B).
 8. An alloy conversion method according to claim 6, wherein the deformation (R5) performed during the third step of the first type (C) is adapted to shorten the length (Lx) of the alloy by 15% to 20% of the length (Lx) of the alloy measured after performing the second step of the second type (B′) and before performing this third step of the first type (C).
 9. An alloy conversion method according to claim 6, wherein the deformation (E2) performed during the first step of the second type (A′) is adapted to increase the length (Lx) of the alloy by 20% to 30% of the length (Lx) of the alloy measured after performing the first step of the first type (A) and before increasing the length (Lx) during this first step of the second type (A′).
 10. An alloy conversion method according to claim 9, wherein the deformation (E4) performed during the second step of the second type (B′) is adapted to increase the length (Lx) of the alloy by 20% to 30% of the length (Lx) of the alloy measured after performing the second step of the first type (B) and before increasing the length (Lx) during this second step of the second type (B′).
 11. An alloy conversion method according to claim 1, wherein after the first step of the first type (C), a third step of the second type (C′) is performed.
 12. An alloy conversion method according to claim 11, wherein after the second step of the first type (B) and before the third step of the second type (C′), a cutting step is performed on a transverse plane of the alloy so as to obtain two elongate portions in the form of bars referred to as billets.
 13. An alloy conversion method according to claim 1, wherein each of the steps of the second type is performed at a fourth temperature (T4) lying between the β transus temperature (Tβ) minus 50° C. to within plus or minus 15° C., and preferably to within plus or minus 5° C.
 14. An alloy conversion method according to claim 1, wherein the alloy is selected to present a β transus temperature (Tβ) lying in the range 800° C. to 950° C., and preferably of 900° C.
 15. An alloy method according to claim 1, wherein the alloy is selected from the group of alloys comprising: a first alloy (Ti 10-2-3) comprising the following elements in percentages by weight: aluminum, Al 2.6%-3.4% carbon, C ≦0.050% hydrogen, H ≦0.015% iron, Fe 1.6%-2.2% nitrogen, N ≦0.050% oxygen, O ≦0.013% titanium, Ti   83%-86.8% vanadium, V 9.0%-11% 

a second alloy of type (Ti 5-5-5-3) comprising, the following elements in percentages by weight: iron, Fe 0.5%-1.5% carbon, C maximum 0.1% silicon, Si maximum 0.15% chromium, Cr 0.5%-1.5% molybdenum, Mo   4%-5.5% vanadium, V   4%-5.5% nitrogen, N maximum 0.05% titanium, Ti 79.4%-86.3% aluminum, Al 4.4%-5.7% zirconium, Zr maximum 0.3% oxygen, O maximum 0.18% hydrogen, H maximum 0.15% impurities 0.3%

a third alloy of type (Ti 5-5-5-3-1) comprising the following elements in percentages by weight: iron, Fe 0.5%-1.5% carbon, C maximum 0.1% silicon, Si maximum 0.15% chromium, Cr 0.5%-1.5% molybdenum, Mo   4%-5.5% vanadium, V   4%-5.5% nitrogen, N maximum 0.05% titanium, Ti 79.4%-86.3% aluminum, Al 4.4%-5.7% zirconium, Zr   1% oxygen, O maximum 0.18% hydrogen, H maximum 0.15% impurities: 0.3%

a fourth alloy of type (Ti 18) comprising the following elements, in percentages by weight: aluminum, Al 5.3%-5.7% vanadium, V 4.8%-5.2% iron, Fe 0.7%-0.9% molybdenum, Mo 4.6%-5.3% chromium, Cr 2.0%-2.5% oxygen, O 0.12%-0.16%

the balance being at least titanium and impurities; and a fifth alloy comprising the following elements, in percentages by weight: titanium, Ti at least 84% aluminum, Al 4%-7.5% oxygen, O at least 0.15% carbon, C at least 0.01%

at least one element selected from vanadium, molybdenum, chromium, and iron, this fifth alloy also including hafnium and zirconium in addition at a percentage by weight of at least 0.1%.
 16. An aircraft landing gear, such as a landing gear rod, a strut, or a truck fabricated from an alloy converted in accordance with the alloy conversion method according to claim
 1. 